General Principles of Tissue Preparation and Staining

The study of tissue structure relies on the preparation of tissue samples in ways that allow their structural details to be viewed at light or electron microscopic levels.

Steps Fixation Processing Sectioning Staining

Fixation

Fixation is used to Terminate cell metabolism. Prevent enzymatic degradation of cells and tissues by autolysis (self-digestion). Kill pathogenic microorganisms such as bacteria, fungi, and viruses. Transform the contents of the cell from a semifluid to a semisolid and prepare the cell contents for visualization with stains..

Fixatives Commonly used fixatives are Formalin (mixture of formaldehyde and alcohol). Aldehydes, such as glutaraldehyde and paraformaldehyde Other fixatives: Picric acid, Alcohols, Mercuric chloride, Acetic acid

Methods of Fixation Immersion fixation Perfusion fixation Fixation is usually accomplished in one of two ways: Immersion fixation Perfusion fixation

Immersion Fixation Small piece of tissue removed and immersed in the fixative. The advantage of this method is that the fixative can penetrate from all sides of the tissue block. This method is useful when the tissue sample is small.

Immersion Fixation

Perfusion Fixation In this method the fixative is perfused through the intact vascular system of the organism. After perfusion, the tissue samples are removed and placed in more of the same fixative used in the perfusion. This method provides superior fixation of large pieces of tissue and is commonly used in many research applications.

Perfusion fixation

FREEZING Is also used as a method of fixation, especially in the clinical setting when a rapid diagnosis is needed during a medical procedure. A fresh tissue sample is retrieved from the patient and immersed in liquid CO2 or in a substance cooled extremely rapidly by dry ice. The best results are obtained with small tissue samples that are rapidly cooled to very low temperatures (–40° to –60° C).

Processing of Fixed Tissue Once the tissue sample is satisfactorily fixed, it must be taken through a series of steps that result in a thin slice of tissue mounted on a glass slide (for LM) or an even thinner slice mounted on a copper grid (for EM). In general, this process requires three basic steps: Dehydration Cleaning Impregnation and Embedding

Dehydration The goal of dehydration is to remove the water from the tissue. The most common method is to start with alcohol in a concentration of 75% to 80%. Recall that alcohol will mix with water; therefore, it can be used to remove water from the tissue. The tissue sample is passed, stepwise, through progressively higher concentrations (from 75%–80% up to 100%) of alcohol. Through this process, the water is completely removed from the tissue and replaced with alcohol.

Clearing The clearing reagent is a substance that will mix both with alcohol and with the embedding medium. The most commonly used are xylene, toluene, and chloroform. The tissue is passed from 100% alcohol through changes of the clearing reagent. This stepwise process progressively removes the alcohol from the tissue and replaces it with the clearing reagent.

Impregnation and Embedding Because the clearing reagent will mix with the embedding medium, the tissue sample is taken from the last step in this reagent and placed in melted embedding medium (paraffin wax). The sample is then progressively passed through several changes of the embedding material. This stepwise process progressively removes the clearing reagent and replaces it with the embedding medium that will harden when cooled.

Embedding Is generally done in two steps. First, the tissue is removed from the last impregnation step and immediately placed in melted medium in a vacuum oven. The last traces of the clearing reagent and any minute bubbles are removed by vacuum. Second, the tissue sample is oriented in an embedding mold. The mold is then filled with melted medium and allowed to cool so the medium hardens

Automated Tissue Processer

Sectioning and Mounting Sectioning of prepared tissues is done on microtomes The sections are cut using extremely sharp metal or glass knives Removed from the edge of the knife either as individual sections or as ribbons of sections, and floated in water (usually warmed). For most applications in LM, the sections range in thickness from about 5 to 12 µm For EM, glass or diamond knives are used to cut extremely thin sections ranges 80 to 110 nm After the sections are cut, they are mounted on slides using a mounting substance as DPX.

Microtome

Staining The goal of staining tissue slices is to use substances to impart color to various components of the section, making these components available for study. The simplest way that stains function is to exploit the electrostatic interactions between the stain molecules and components of the cell; positive charges on cellular structures attract negatively charged stain molecules and vice versa.

Basic dyes Carry positive charges and are, consequently, known as cationic dyes; they are attracted to negative charges within the tissue. Hematoxylin and toluidine blue are commonly used basic (cationic) dyes. They stain nuclear DNA, cytoplasmic RNA.

Acidic dyes Carry negative charges and are, consequently, known as anionic dyes; they are attracted to positive charges within the tissue. Eosin Y is a commonly used acidic (anionic) dye. It stains many proteins (and, therefore, stains many structures within the cell), and acid dyes also stain extracellular structures such as collagen.

Commonly used stains Hematoxylin and Eosin (H&E) is, by far, the most commonly used combination stain. This combination of cationic and anionic dyes results in most constituents of the cell (RNA, DNA, polysaccharides, and others) being stained various tones of either blue or pink. This method also stains extracellular collagen. The Mallory trichrome stain Wright stain Silver stains The periodic acid-Schiff (PAS) reaction

Immunocytochemistry Immunocytochemistry is a specialized method that can be used to precisely localize enzymes or large molecules (macromolecules) within the cell or on its membrane. The immune system of the body is able to defend itself against foreign molecules (antigens) by producing specific types of proteins (antibodies).